The Effects of Acetyl-L-Carnitine Supplementation

Myra J. Shapiro

Carnitine is the generic term used to refer to both acetyl-L-carnitine and L-carnitine interchangeably. Acetyl-L-carnitine is the delivery form of L-carnitine, an amino acid derivative synthesized from the essential amino acids lysine and methionine. While L-carnitine is found in almost all cells in the human body, the majority of the L-carnitine pool is concentrated in skeletal and cardiac muscles. Acetyl-L-carnitine transports L-carnitine formed in the liver and kidneys or absorbed in the small intestine from foods through the blood for delivery to muscles and tissues. Carnitine is classified as a “conditionally essential” nutrient meaning that under certain conditions the body’s requirement exceeds production capability. Dietary carnitine is primarily obtained from meat and dairy products; only trace amounts can be found in most plant foods with a few exceptions. As a result, vegetarians and vegans may need to rely on supplementation to ensure that the fundamental level of carnitine necessary for bodily function is met. Elevating carnitine levels through supplementation can also be beneficial for athletes interested in enhancing fat burning and improving performance.
One of the primary functions of carnitine in the body is the regulation of fat metabolism. Fatty acids, the form of fat used by the body for fuel, cannot be metabolized without carnitine. Research has demonstrated that the availability of carnitine is a limiting factor in the utilization of fat as energy during exercise. In addition, very low calorie diets have been shown to lower levels of carnitine in the blood and bodily tissues. For individuals trying to lose fat, especially athletes in weight restricted sports, lower levels of carnitine in the body may prove to be detrimental to weight loss. Raising carnitine levels can also benefit athletes by increasing their potential to train at a greater intensity and prolonging time to fatigue. During exercise, carnitine causes a sparing effect on muscle glycogen by enhancing fat utilization. Glycogen, the storage form of carbohydrates in muscle, provides a pool of energy that plays a vitally important role in exercise performance. Muscle glycogen depletion is often the root of fatigue during exercise and increased fat burning can prolong time to exhaustion. Scientists have shown that carnitine may also increase maximal aerobic performance in endurance athletes.
Carnitine has also been shown to have antioxidant properties in cells that provide a bolstering effect on the immune system. Research supports the notion that supplemental carnitine has a protective effect on the heart and cardiac system. Carnitine supplementation has shown to lower risk factors in heart disease by lowering triglyceride levels and raising HDL cholesterol—the “good” cholesterol that scavenges artery clogging plaques. Scientists have also discovered that carnitine supplements can have an anti-aging effect in both skeletal muscle and brain function. In skeletal muscle, carnitine effects the alterations in stress response during aging and inhibits the decline of physiological function. Ongoing research on the relationship between carnitine on brain function has led to the conclusion that acetyl-L-carnitine may be neuroprotective agent that can defend against strokes and Alzheimer’s disease.
Recent research on the role of carnitine in men’s health has demonstrated a positive effect on reproductive function. Supplemental carnitine has been shown to play a beneficial role in male fertility by increasing sperm production and providing a line of antioxidative defense in the reproductive system.
Scientists have found muscle carnitine levels to decrease with training making carnitine supplementation a good idea for all athletes. Acetyl-L-carnitine is the preferred form of supplementation because it is more readily absorbed by the body and available for faster transport to muscle and tissues than L-carnitine. In addition, the protection against heart disease, aging and benefits in immune system function may give athletes a better sense of overall well being.
It should be noted that athletes should never take carnitine in the D-carnitine or D,L-carnitine (a mixture of the D-carnitine and L-carnitine) forms. Supplements containing D-carnitine have demonstrated a toxic effect in humans and have been outlawed in the United States. The D-carnitine compound has also been shown to interfere with the function of both L-carnitine and acetyl-L-carnitine thereby undermining any potential benefits of carnitine supplementation.

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